This invention relates to the recovery of petroleum from underground reservoirs and pertains in particular to the breaking of emulsions of crude oil and water that are recovered from a producing well of the reservoir in an enhanced recovery process.
Because oil is generally in short supply, petroleum technologists have tried to extract a greater amount of the oil in place in a given reservoir. After a primary recovery has been completed, secondary recovery, or water injection, is frequently begun to displace more of the oil. Even after no more oil can be displaced from a formation by water injection there is frequently still quite a lot of oil in the ground which can be recovered by a tertiary recovery operation. Most tertiary recovery projects use surface active agents to scrub the oil out of the rocks or sands in which the oil is found, permitting displacement of the oil to producing well.
The surfactants which are added, usually petroleum sulfonates or derivatives thereof, have a built-in defect. Surface active agents which are efficient at removing petroleum from a reservoir are frequently exceedingly difficult to remove from the petroleum once it has been produced.
In most enhanced recovery processes, significant amounts of water will be produced, along with the oil, usually in the form of very stable emulsions. Breaking of emulsions produced in a tertiary recovery operation is difficult because the surfactant stabilize the emulsions. Frequently, there is not much oil present in the emulsions, from 1-30 lv% oil includes most of the emulsions to be produced, with 10-20 lv% oil being more commonly encountered as the surfactant is being produced.
The emulsions are oil-in-water, or o/w, emulsions. These emulsions are not usually encountered in petroleum production where the expected form is a water-in-oil, w/o, emulsion. Conventional emulsion breaking techniques which work on a w/o emulsion are ineffective in breaking an o/w emulsion.
Compounding the difficulties of enhanced recovery processes, the surfactants used are relatively expensive, and add greatly to the cost of such processes. The surfactants, which tend to stay in the oil phase, may cause problems in downstream processing units. To the extent that the surfactants do not end up in the crude, they are pollutants. Disposal of aqueous streams containing these surface active agents in streams and other bodies of natural water causes pollution. Most significant is the loss of a valuable material, the surfactants, which could be reused in the surfactant flood, or elsewhere.
In surfactant flooding, oil is hard to produce, hard to separate from the water used to produce it, and hard to live with in downstream processing operations. The great expense and difficulties of these processes have all acted to slow the development of surfactant recovery processes in most countries.